1. Field of the Invention
The present invention relates generally to a magnetic tunnel junction structure for a magnetic random access memory (MRAM), and in particular, to a magnetic tunnel junction structure with an amorphous CoFeSiB or NiFeSiB free layer.
2. Description of the Related Art
A magnetic random access memory (hereinafter, referred to as an MRAM) has been used in military applications, such as missiles and spaceships. In addition, there is a strong possibility that the MRAM will be used as a substitute for an existing memory of portable phones, computers, and so on. This MRAM is a magnetic storage device that has magnetic tunnel junctions (MTJs) based on a tunneling magnetoresistance (TMR).
An MTJ structure includes an insulating layer (generally Al2O3 or MgO) and two ferromagnetic layers. The insulating layer serving as a tunneling barrier is interposed between the two ferromagnetic layers. A current flows vertically to the respective layers. The two ferromagnetic layers are a pinned layer acting as a reference layer and a free layer performing a storing and sensing function. When a current flows, if the spin orientations of the two ferromagnetic layers are equal to each other, a parallel resistance is small and a current tunneling probability increases. On the contrary, when the spin orientations are antiparallel, a resistance is large and a current tunneling probability decreases. That is, a tunneling current in the MTJs depends on a relative magnetization orientation of the two ferromagnetic layers. This phenomenon was first observed experimentally by Julliere in 1975. This phenomenon is referred to as a tunneling magnetoresistance (TMR).
MTJs have a promise in high density read head and MRAM applications because they exhibit large TMR ratios (J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, Phys. Rev. Lett. 74, 3273 (1995), W. J. Gallagher, S. S. P. Parkin, Yu Lu, X. P. Bian, A. Marley, K. P. Roche, R. A. Altman, S. A. Rishton, C. Jahnes, T. M. Shaw, and Gang Xiao, J. Appl. Phys. 81, 3741 (1997)).
In the MRAM, the core technology in device implementation is the development of thin film materials exhibiting excellent and stable magnetoresistance properties, and the integration process using existing semiconductor circuits. From these points of view, magnetoresistance films (i.e., MTJs) exhibiting TMR phenomenon are considered as film materials that are most suitable for the development of excellent non-volatile MRAMs.
Accordingly, the interactions between the two ferromagnetic (pinned and free) layers are very important particularly in the switching characteristics of the high density MRAM with submicrometer-sized MTJ cells. A smaller cell, however, requires a larger switching field (HSW) because a large demagnetizing field arises from the poles of the submicrometer-sized cell edge, leading to large write currents. That is, a writing margin needs to be assured in forming the submicrometer-sized cells. When the size of the cell is reduced to less than micrometers, a demagnetizing field is generated so that the writing margin is reduced. The writing margin is greatly influenced by a physical property of the ferromagnet itself. In order to assure the writing margin, a low switching field is required.
In addition, a reading margin needs to be assured in forming the submicrometer-sized cells. The reading margin is greatly influenced by a TMR ratio. In order to assure the reading margin, a large TMR ratio is required.
Accordingly, as the size of the cell is reduced, MTJs need to have a low switching field and a large TMR ratio. In addition, a free layer for these MTJs must be provided.